mkroot: Print when we're syncing the output file

Provides a bit better feedback on what we're doing
First cut of directory indexing

We create a tmp file to store content for the indexes and just append
each index page to the end of that. The tmpfile is pre-deleted to ensure
cleanup if the mkroot process is terminated unexpectedly.

Then we just indicate in the file records whether we are to read a
file or read a section out of the tmpfile.

The path_splice() code was modified to prevent extraneous slashes
when we construct the links in the indexes. Seems like a good idea
anyway.

Also, write_files() was modified to fix the condition where we end
up with a bogus webroot in some instancess if files were modified
during the process. However it still breaks if files are truncated
downwards so we probably ought to detect that condition later.
Print a helpful error if files were shrunk during mkroot

Also, abort the process as we cannot continue in this circumstance.